StringExtensions.cs

using System;
using System.Security.Cryptography;
using System.Text;

namespace Example
{
  public static class StringExtensions
  {
    /// <summary>
    /// Hash a string with SHA-1.
    /// </summary>
    /// <param name="self">
    /// The string to hash.
    /// </param>
    /// <returns>
    /// The hashed string.
    /// </returns>
    public static string Sha1(this string self)
    {
        return self.Sha1(Encoding.Default);
    }

    /// <summary>
    /// Hash a string with SHA-1, encoding in the given encoding.
    /// </summary>
    /// <param name="self">
    /// The string to hash.
    /// </param>
    /// <param name="encoding">
    /// The encoding to decode the supplied string.
    /// </param>
    /// <returns>
    /// The hashed string.
    /// </returns>
    public static string Sha1(this string self, Encoding encoding)
    {
        if (self == null)
        {
            throw new ArgumentNullException("self");
        }

        if (encoding == null)
        {
            throw new ArgumentNullException("encoding");
        }

        return BitConverter.ToString(new SHA1CryptoServiceProvider().ComputeHash(encoding.GetBytes(self)))
            .Replace("-", string.Empty)
            .ToLower();
    }
  }
}